Process for obtaining lignin from alkaline solutions thereof

Abstract

The invention relates to a method and apparatus for recovering lignin and kali from an alkaline lignin solution. One or more electrolytic cells are used to anodically acidify the lignin and simultaneously cathodically regenerate alkali. The invention is especially advantageous for the preparation of pure lignin and alkali from the waste liquor of a cellulose process.

What is claimed is: 
     
       1. A process for obtaining lignin from an alkaline lignin solution, comprising: anodically acidifying the alkaline lignin solution by electrolysis;   precipitating lignin from the solution; and   cathodically regenerating and recovering alkali from the solution.   
     
     
       2. The process of claim 1, wherein the process is carried out continuously. 
     
     
       3. The process of claim 2, further comprising: feeding an alkaline lignin solution to the anode chamber of an electrolytic cell containing anode and cathode chambers, separated by a cation exchange membrane;   anodically acidifying the solution in the anode chamber, and forming a precipitate of lignin foam thereby;   simultaneously regenerating the alkali in the cathode chamber after ion exchange of alkali ions across the membrane;   separating the lignin foam into lignin and an acidic anolyte; and   recovering the lignin and the alkali.   
     
     
       4. The process of claim 3, wherein the process is carried out at a temperature slightly less than the boiling point of the alkaline solution. 
     
     
       5. The process of claim 4, wherein the alkaline lignin solution is an extract or waste liquor from a cellulose process. 
     
     
       6. The process of claim 5, wherein the acidic anolyte obtained in separating the lignin foam is recycled to the cellulose process. 
     
     
       7. The process of claim 6, wherein the alkali is sodium hydroxide. 
     
     
       8. The process of claim 7, wherein the alkaline lignin solution is the extract from an organosolv process for the production of cellulose, from which the organic solvent has first been removed. 
     
     
       9. The process of claim 8, wherein dilute alkaline liquor, water, or the acidic anolyte is fed to the cathode chamber. 
     
     
       10. The process of claim 9, wherein the lignin foam separating step includes forming a lignin suspension by flotation. 
     
     
       11. The process of claim 9, wherein the lignin foam separating step includes centrifuging to form pure lignin and a centrifugate comprising the acidic anolyte. 
     
     
       12. A multistage process for obtaining lignin from an alkaline lignin solution, comprising: anodically acidifying the alkaline lignin solution by electrolysis, in a first stage, to a pH corresponding to the onset of lignin precipitation;   thereafter further acidifying the solution in succeeding stages to effect precipitation of the lignin.   
     
     
       13. The process of claim 12, wherein the process is carried out continuously. 
     
     
       14. The process of claim 13, wherein the process has two stages. 
     
     
       15. The process of claim 14 including: feeding an alkaline lignin solution to the anode chamber of a first electrolytic cell containing anode and cathode chambers separated by a cation exchange membrane;   anodically acidifying the solution in the anode chamber to a pH corresponding to the onset of lignin precipitation, and forming a first anolyte solution thereby;   simultaneously regenerating the greater portion of the alkali in the cathode chamber after ion exchange of alkali ions across the membrane, and forming a first catholyte solution thereby;   passing the first anolyte solution from the anode chamber of the first cell to the anode chamber of a second electrolytic cell having anode and cathode chambers separated by a cation exchange membrane;   further anodically acidifying the first anolyte solution in the anode chamber of the second cell and forming a precipitate of lignin foam thereby;   simultaneously further cathodically regenerating alkali from the first anolyte solution in the cathode chamber of the second cell after ion exchange of alkali ions across the cation exchange membrane, forming a second catholyte solution thereby;   separating the lignin foam into lignin and a second anolyte solution; and   recovering the lignin.   
     
     
       16. The process of claim 15, further including recycling the second anolyte solution to the cathode chamber of the second electrolytic cell; passing the catholyte from the cathode chamber of the second cell to the cathode chamber of the first cell; and   recovering the alkali from the catholyte of the first cell as an alkali solution.   
     
     
       17. The process of claim 15, wherein the process includes recycling the second anolyte solution to the catholyte chamber of the first electrolytic cell, and recovering the alkali from the catholyte of the first cell as an alkali solution. 
     
     
       18. The process of claim 16, wherein the alkaline lignin solution is an extract or waste liquor from a cellulose process. 
     
     
       19. The process of claim 18, wherein the process includes adjusting the concentration of the recovered alkali solution and recycling the alkali solution to the cellulose process as the digesting medium, or a component thereof. 
     
     
       20. The process of claim 19, wherein the alkaline lignin solution is anodically acidified in the first electrolysis cell to a pH of about 9.5. 
     
     
       21. The process of claim 20, wherein the first anolyte solution is anodically acidified in the second electrolysis cell to a pH of about 4. 
     
     
       22. The process of claim 21, wherein the process is carried out at a temperature slightly less than the boiling point of the alkaline lignin solution. 
     
     
       23. The process of claim 22, wherein the lignin foam separating step includes forming a lignin suspension by flotation. 
     
     
       24. The process of claim 23, wherein the process includes continuously circulating the first anolyte and catholyte solutions by passing the solutions from the anode and cathode chambers of the first cell separately through respective gas separators for removal of anodically and cathodically generated oxygen and hydrogen gases, passing the solutions from the gas separators to respective reservoirs, and returning the solutions to the anode and cathode chambers of the first electrolytic cell. 
     
     
       25. The process of claim 24, wherein the alkali concentration of the first and second catholytes is adjusted by adding thereto additional alkali or water or both. 
     
     
       26. The process of claim 25, wherein the alkali concentration adjustment to the first catholyte is made at the catholyte reservoir. 
     
     
       27. The process of claim 26, wherein the alkali concentration in the second catholyte is adjusted to a value consistent with minimum conductivity of the catholyte. 
     
     
       28. The process of claim 27, wherein the alkali is sodium hydroxide.